Whether it is baseball, football, or martial art training, all training requires a lot of good repetitious practice.  If one spars or participates in sparring of any kind, that individual is required to strike targets that are, frankly, less vulnerable – like kicking the body or limiting blows to the limbs  to targets like the thicker, muscular thighs.  Getting good at doing that, means they have developed the needed habits to actually be good at it.  Such is proof that what they do in their training is just what they will do automatically in any fight.  Good for them!  (With all that kind of training they also develop very strong and fast blows.)  Again, good for them.  It also means that they will never reflexively strike their opponent's knees, thereby keeping their training and sparring partner's relatively safe – hence, the reasons for the required rules.

Training For Effective  Self-protection
That said,  when one trains for effective self-protection  – what we define as destroying an assailant's ability to hurt and seriously injure us or those we must protect – then practicing strikes to less vulnerable targets is definitely unwise there. 

Because of the danger inherent in the techniques we teach and practice (e.g., arm breaking, knee smashes to the legs, kicks to the knees and groin, to name a few), sparring, in the sport martial arts sense, is simply too dangerous for use in the light of what we practice, teach, and train.  However, sparring-like drills – drills that develop the ability to close on a moving opponent – are a regular part of our training regimen.

1.  When you bite, bring the teeth together  (take out a good DNA sample for the cops).
2.  Neither box with a boxer, nor grapple with a grappler.
3.  Never fight an opponent's strength – attack his vulnerabilities and weaknesses.
4.  General: Rush a gun;  RUN  from a knife.
5.  General: Once the fight begins, keep your lead leg shin oriented on opponent's near leg.